1. Field of the Invention
The present invention relates to a braking system for a vehicle, and, more particularly, to a braking system for a vehicle, which can prevent an alteration in brake pedal toe force when a brake hydraulic pressure is increased or decreased based on a regenerative braking force obtained by a drive motor in an electric vehicle or hybrid vehicle, and a braking method thereof.
2. Description of the Related Art
Generally, in operation of a braking system, if a vehicle operator presses a brake pedal, a pressure corresponding to a boost ratio of a booster is generated in a master cylinder based on a pedal toe force applied by the vehicle operator. As the pressure is transmitted to each wheel brake device through a valve, a wheel braking operation is realized. Differently from the above described conventional braking system, an electric vehicle or hybrid vehicle employs a drive motor as a electric generator, and uses a regenerative braking force as well as a frictional braking force to brake a vehicle.
Recently, restrictions related to exhaust gas have been tightened to prevent air pollution by exhaust gas of vehicles. Accordingly, to reduce the discharge amount of exhaust gas in response to the restrictions, electric vehicles using electricity as drive power or hybrid vehicles using electricity and gasoline together are becoming increasingly more widely used.
Electric vehicles include a high-capacity drive motor and a battery for storing electric energy, and are driven as the battery is charged and the drive motor is operated using the electric energy charged in the battery. The electric vehicles are clean vehicles completely free from the discharge of exhaust gas and are increasingly used to prevent air pollution. Hybrid vehicles are a cross between oil-powered vehicles and electric vehicles, and can be referred to as transient vehicles being spread at low costs prior to high-cost electric vehicles becoming more common.
In the case of the electric vehicles or hybrid vehicles, if the vehicle operator presses a brake pedal, a resulting pedal operating speed and pedal toe force are detected by a pedal travel sensor, and thus, a brake hydraulic pressure corresponding thereto can be calculated and generated by a hydraulic pump, etc. The electric vehicles or hybrid vehicles have a drive motor for the traveling of vehicles. During a speed-reduction or braking operation, the drive motor functions as a generator using a reverse torque of vehicle wheels, thereby serving to charge the battery. Also, the drive motor generates a regenerative braking force during the speed-reduction or braking operation. The regenerative braking force depends on various factors, such as the charge state of the battery, the speed of a vehicle, etc. Under the presence of the regenerative braking force, a brake hydraulic pressure has to be increased or decreased to fulfill the amount of braking desired by the vehicle operator.
FIG. 2 is a graph showing variations of frictional braking force and regenerative braking force depending on time. In an initial state of braking, a vehicle can be braked sufficiently only by the amount of regenerative braking. In this case, a brake hydraulic pressure is decreased by directing a hydraulic pressure generated by a vehicle operator's pedal toe force to other sites, thereby allowing the vehicle to be braked sufficiently only by the regenerative braking. However, in the case where such a decrease in brake hydraulic pressure is realized by brake oil in a master cylinder, there is a problem in that a brake pedal is depressed unintentionally due to a decreased pressure, thereby providing the vehicle operator with unusual pedal feeling. Although an electro-hydraulic brake (EHB) may be used to keep a braking pressure and pedal pressure constant, the EHB has problems of complicated structure and high costs thereof, and suffers from deterioration in braking reliability/durability.